DC voltage converters are used in portable devices in which the batteries provide only a low supply voltage on account of the desired miniaturization and weight saving. In order to supply the circuit units of the devices, a DC voltage converter converts the supply voltage into a higher output voltage. The design of such a DC voltage converter (which is also referred to as a step-up converter) is described in Tietze/Schenk: “Halbleiterschaltungstechnik” [Semiconductor circuit technology], 12th edition, pages 948 to 949.
A DC voltage converter has an inductive storage element, which is connected between a terminal for the supply voltage and, in such a manner that it is coupled via a first switch, a terminal for the reference potential. A capacitively buffered terminal for the output voltage is connected, via a second switch, between the inductive storage element and the first switch.
During ideal cyclic operation, the first switch and the second switch change over simultaneously, with the result that either the first switch is on and the second switch is off or the first switch is off and the second switch is on, respectively. If the first switch is on, energy is stored in the inductive storage element. The charge is removed if the second switch is on, and the capacitor is charged. If the switching states of the first and second switches remain unchanged, the coil current falls continuously until the inductive storage element has been discharged.
If the first and second switches are on, the energy stored in the capacitor drains via the second and first switches. This impairs the efficiency of the DC voltage converter.
If the first and second switches are off at the same time and energy is still stored in the inductive storage element, the coil current is interrupted and voltage spikes which may damage the circuit occur. In addition, interfering oscillations occur, said oscillations being caused by the resultant resonant circuit that is formed from the inductive storage element and the parasitic switch capacitance.
In practice, the first and second switches cannot be changed over at exactly the same time on account of propagation times and other effects. Therefore, it is difficult to avoid the first and second switches being off at the same time. The first and second switches may likewise be on at the same time. This state is usually avoided on account of the efficiency being greatly reduced. In addition, it is also sometimes desired, when regulating the circuit, that the first and second switches are off at the same time so that the output voltage assumes a prescribed value. In order to suppress the associated voltage spikes or voltage oscillations, the inductive storage element is shorted, with the result that the coil current flows in the short-circuit circuit. Such a circuit is also referred to as a snubber circuit. A snubber circuit has hitherto usually been formed by a series circuit comprising a diode and a series resistor, said series circuit being connected in parallel with the inductive storage element. The series resistor is used to set a voltage value, at which the inductive storage element is shorted. The voltage value may not be less than the threshold voltage of the diode, which, in integrated circuits, is possibly already a critical value.
The problem is that both the diode and the resistor must be dimensioned as accurately as possible (which is associated with outlay in terms of technology and costs) so that, on the one hand, the inductive storage element is shorted at a desired threshold value and, on the other hand, the losses on account of the voltage drop across the resistor are not too large.